In recent years, in thin film transistor substrates included in, for example, liquid crystal display panels, as switching elements of pixels each of which is a minimum unit for forming an image, thin film transistors (hereinafter referred to as “TFTs”) which use semiconductor layers made of an oxide semiconductor (hereinafter also referred to as “oxide semiconductor layers”) and have satisfactory characteristics such as high mobility, high reliability, and low off currents have been proposed instead of conventional TFTs using semiconductor layers made of amorphous silicon.
For example, Patent Document 1 discloses methods for fabricating a field effect transistor, wherein the methods include: a (first) method in which before forming an active layer containing amorphous oxide on a substrate, irradiating a surface of the substrate with ultraviolet light in an ozone atmosphere, irradiating the surface of the substrate with plasma, or cleaning the surface of the substrate with a chemical solution containing hydrogen peroxide is performed; a (second) method in which the forming the active layer is performed in an atmosphere containing at least any of ozone gas, nitrogen oxide gas, oxygen-containing radicals, atomic oxygen, oxygen ions, or oxygen radicals; a (third) method in which after the forming the active layer, a thermal treatment at a temperature higher than a film formation temperature of the active layer, or irradiating the substrate provided with the active layer with oxygen plasma is performed; a (fourth) method in which the forming the active layer is performed by resistance heating evaporation, electron beam evaporation, chemical vapor deposition, line beam laser evaporation, or electrodeposition; and a (fifth) method in which the forming the active layer is performed at a film formation temperature higher than or equal to 70° C.